For those of you who worked before deciding to become SAHMs, how did you tell your boss that you weren't planning to come back? Did you explain any of your reasoning behind your choice, or just state that that's what you were doing?

I am due with my first in June and work in a teeny tiny academic research lab. It is just my boss, me, and one other girl. I found out this morning that my one other coworker just put in her two weeks notice and I am planning to not come back after the birth of my baby this summer.

DH and I have decided to let my boss know this on Monday so that he can make re-hiring decisions accordingly versus thinking that I'm coming back which might change his plans. So I'd really appreciate any insight on how the conversation went when any of you told your bosses that you would not be returning.